Couple of things. Check the ground for the loom at the rear. If that is clean and tight I would suspect that the problem is in the brake light switch.
brake hose's are about $20 each , steel lines can be replaced in sections for about $5 a section
How to change the brake switch on a 2004 Jeep grand Cherokee Laredo
Warped brake rotors. This is a very common problem for the 1999-2004 Grand Cherokees. You will need to have your rotors replaced, or resurfaced. The best resurface method is with an on car brake lathe.
Your brake booster is malfunctioning
If equipped, it is above the liftgate glass.
brake hose's are about $20 each , steel lines can be replaced in sections for about $5 a section
The brake pads might not be adjusted properly. The brake pads can be too tight and have contact with the rotors.
That would depend on what the problem is and if they can be repaired or have to be replaced. If they can be repaired it can be as little as $20 per side or up to $300 if everything has to be replaced.
How to change the brake switch on a 2004 Jeep grand Cherokee Laredo
When Brake Rotors Are To Thin To Turn On A Brake Lathe To Have The Minumn Thickness. They Need To Be Replaced.
The emergency or parking brake on 2003 Jeep Grand Cherokee operates a set of brake shoes inside a hub on the rear axles, this is incorporated into the disc brake setup.
Did anybody bleed the brake lines? There is possibly still air trapped in lines and air compresses brake fluid does not.
Warped brake rotors. This is a very common problem for the 1999-2004 Grand Cherokees. You will need to have your rotors replaced, or resurfaced. The best resurface method is with an on car brake lathe.
Most often a brake repair simply involves replacing the brake pads. If the pads have worn too much though the rotors or drums will also need to be resurfaced or replaced. In more serious cases the master cylinder that controls the brake pressure will be replaced. Brake lines rarely if ever need to be replaced.
The brake light switch is behind the brake pedal.
If you did not have drums re-surfaced linings may have to be burnished (wear in) for a few hundred miles prior to obtaining full braking effect
You have a "Warped" brake rotor. It needs to be replaced